C++ :

#include <cstdio>
int main(void) {
    int K, i, j, h[2]= {4,7};
    scanf("%d", &K);
    for(i=1, j=2; K>j; ++i, j*=2) K -= j;
    for(;i>0;--i, K/=2) printf("%d", h[K/2>0]);
    return 0;
}

Pascal :

var k,i,j:longint;a:array[1..50000]of 0..1;
begin
 readln(k);
 k:=k+1;
 i:=0;
 repeat
   inc(i);
   a[i]:=k mod 2;
   k:=k div 2;
 until k=1;
 for j:=i downto 1 do
  if a[j]=1 then write(7) else write(4);
 writeln;
end.